The present invention relates generally to automotive HVAC systems for controlling the environment of an automobile passenger compartment. More particularly, the invention relates to a system for controlling the operating modes in an automotive HVAC system.
This application is related to co-pending applications all filed on Nov. 12, 1998 and titled Refrigerant Flow Management Center For Automobiles, Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Anti-Fog Controller For Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Controller For Heating In Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Air Handling Controller For Hvac System For Electric Vehicles, and System For Cooling Electric Vehicle Batteries. Each of these applications is incorporated by reference into the present application.
Automotive climate control systems have traditionally been single loop designs in which the full volume of refrigerant flows through each component in the system. In an HVAC system, refrigerant in the vapor phase is pressurized by a compressor or pump. The pressurized refrigerant flows through a condenser which is typically configured as a long serpentine coil. As refrigerant flows through the condenser heat energy stored in the refrigerant is radiated to the external environment resulting in the refrigerant transitioning to a liquid phase. The liquefied refrigerant flows from the condenser to an expansion valve located prior to an evaporator. As the liquid flows through the expansion valve it is converted from a high pressure, high temperature liquid to a low pressure, low temperature spray allowing it to absorb heat. The refrigerant flows through the evaporator absorbing heat from the air that is blown through the evaporator fins. When a sufficient amount of heat is absorbed the refrigerant transitions to the vapor phase. Any further heat that is absorbed pushes the vaporized refrigerant into the superheated temperature range where the temperature of the refrigerant increases beyond the saturation temperature. The superheated refrigerant flows from the outlet of the evaporator to the compressor where the cycle repeats. Generally, the refrigerant flowing into the compressor should be in the vapor phase to maximize pumping efficiency. The operation of the refrigerant loop in conventional automotive HVAC systems is controlled by cycling the compressor on and off, and by varying the volume of refrigerant that is permitted to flow through the expansion valve. Increasing the volume of refrigerant that flows through the valve lengthens the distance traversed by the liquid before it changes to the vapor phase, allowing the heat exchanger to operate at maximum efficiency.
Advances in automotive HVAC systems have led to zone temperature control systems wherein different zones of an automobile are independently controlled. Zone control systems generally include an evaporator and expansion valve for each zone. Generally, the refrigerant flows through a compressor and condenser, then is split by a system of valves before flowing to the expansion valve and evaporator of each zone. The refrigerant flowing out of the evaporator of each zone is then recombined before returning to the compressor. A complex series of valves and plumbing is generally required to maintain a balanced HVAC system that provides individualized control for each of the zones. The refrigerant plumbing associated with zone control systems is significantly more complex than the plumbing of prior single loop designs.
The complexity of refrigerant plumbing has further increased with the recent implementation of reversible heat pump systems in automobiles. In a reversible heat pump system the HVAC system can either heat or cool a compartment depending on the direction of the refrigerant flow. In the air conditioning mode refrigerant flows from the compressor through an outside coil (condenser) and into an expansion valve and inside coil (evaporator) before returning to the compressor. Heat energy is extracted from air that is blown through the inside coil (evaporator) into the passenger compartment thus providing cooled air. In the heating mode a four way valve reverses the flow of refrigerant through the coils, thereby reversing the function of the coils. Refrigerant flows from the compressor through the inside coil (condenser) then into an expansion valve and the outside coil (evaporator) before returning to the compressor. Heat energy in the liquefied refrigerant flowing through the inside coil is absorbed by air that is blown through the coil into the passenger compartment thus providing heated air. For the reversible system to operate, valves with associated plumbing must be provided to bypass one of the expansion valves during each mode. When zone control is added to a reversible heat pump system the complexity and cost of the HVAC further increases. In addition to excessive cost the system becomes less reliable due to the increased number of valves, plumbing, and control software required for the system.
A solution to this problem is presented in co-pending U.S. patent application titled Reversible Air Conditioning And Heat Pump Hvac System For Electric Vehicles and filed on Nov. 12, 1998. The disclosed invention simplifies the interconnection of automotive HVAC systems by providing a multi-port flow management device to replace the complex nest of valves and plumbing required in conventional systems. When the HVAC system is in the air conditioning mode, refrigerant flows into one of the ports of the flow management device then through a single expansion valve and out the other input/output port. When the system reverses the refrigerant flow and enters heat pump mode the refrigerant flows into the input/output port it previously flowed out of, then flows through the expansion valve and out the other port. By providing bi-directional input/output ports the disclosed invention eliminates the complex plumbing and extra valves associated with conventional systems. In addition, the disclosed invention can further integrate the receiver/drier function into the flow management device providing a centralized flow management center for tapping off refrigerant flow for secondary zone control cooling loops.
Generally, conventional systems control the operation of HVAC systems by regulating the expansion valve and the compressor. The volume of saturated refrigerant emitted from the expansion valve is regulated to ensure the refrigerant transitions to the vapor state at the outlet of the evaporator. As atomized refrigerant is emitted from the expansion valve its temperature rapidly decreases to the saturation temperature corresponding to the compressor suction pressure. The temperature remains relatively constant as the refrigerant traverses the evaporator accumulating heat until sufficient heat is absorbed to push the refrigerant into the vapor state. Any further heat that is absorbed causes the vapor to become superheated, raising its temperature above the saturation temperature. Conventional mechanical expansion valves route the evaporator inlet and outlet lines through the valve to measure the temperature of the refrigerant at the inlet and outlet, and to control the flow at the inlet. The inlet temperature provides the saturation temperature at the system low side pressure. The temperature of the refrigerant at the outlet is used as the controlled input. The expansion valve is dynamically controlled such that the evaporator outlet temperature is a predetermined number of degrees hotter than the outlet saturation temperature. This ensures that the refrigerant has changed to the vapor state and become superheated, thereby eliminating the risk of flooding the compressor. Routing the evaporator inlet and outlet lines through the expansion valve requires one of those lines to doubleback from its normal routing, thereby increasing the complexity of the plumbing required for the HVAC system. Also, sensing temperature provides a much slower responding system than sensing pressure. Therefore, using a temperature probe to directly sense the saturation temperature of the refrigerant results in sluggish system response to changes in the compressor suction pressure. In conventional systems using a compressor thatis cycled on and off the slow response delays the system start-up but has only a minimal effect during steady-state operation. With electric compressors that are controllable over a wide range of suction pressures the slow expansion valve response impacts steady-state response as well as start-up.
Generally, the compressor in internal combustion vehicle HVAC systems is cycled on and off, therefore to control the air temperature blown into the passenger compartment the compressor is cycled on and off at a rate that provides air from the evaporator that is slightly cooler than requested. A PTC heater located downstream from the evaporator heats the air to the requested temperature. With the introduction of electric compressors to automotive systems the air is still generally cooled to a temperature that is slightly cooler than requested, then heated with a downstream PTC heater. The temperature of the air blown into the passenger compartment is sensed by a thermister and compared to a desired temperature that is inputted by the vehicle passenger. The error between the desired temperature and the measured temperature is used to vary the speed of the compressor. Attempting to control the passenger compartment temperature directly results in a sluggish system response wherein there is an excessive delay before the desired steady-state temperature is attained or the system does not settle out to a stable temperature, but instead oscillates about the desired temperature; first supplying air that is to hot, then supplying air that is to cold.
The present invention provides an improved means of controlling an HVAC system by using suction pressure as an indirect measure of the air temperature flowing out of an evaporator. By controlling suction pressure, faster system response to changes in the desired temperature or system operating conditions is attained. A faster responding system provides the vehicle passengers with appropriately cooled air at a stable temperature in less time than conventional systems. Passenger comfort is enhanced by minimizing the time they have to endure sitting in sweltering heat before the air from the air conditioner is sufficient to cool them.
One object of the present invention is to provide a system for improving the steady-state response time of an automotive HVAC system.
Another object of the present invention is to decrease the start-up time of an automotive air conditioning system.
It is an additional object of the present invention to provide a system for controlling an HVAC system that employs a flow management device.
A further object of the present invention is to provide a system for controlling an HVAC system incorporating a centralized flow management center.
Accordingly, the invention provides a control system for controlling a reversible HVAC system of a motor vehicle. The HVAC system includes an electric compressor for circulating a refrigerant through a heat pump. The speed of the compressor is controlled by a compressor control signal having a variable duty cycle. Inside and outside heat exchangers transfer heat energy between an outside environment and a passenger compartment of the motor vehicle. A refrigerant flow switching device switches the direction of refrigerant flow towards the inside heat exchanger in a heating mode and towards the outside heat exchanger in a cooling mode. A pressure reducing assembly supplies pressure reduced refrigerant to the inside heat exchanger in the cooling mode and to the outside heat exchanger in the heating mode. The pressure reducing assembly includes an electronic expansion valve in which the flow rate is set by an EXV control signal having a variable duty cycle. A controller controls the duty cycle setting of the compressor control signal and the EXV control signal such that a desired quantity of heat is transferred between the outside environment and the passenger compartment.
The above described device is only an example. Devices in accordance with the present invention may be implemented in a variety of ways.